Keep the River Flowing - Plastic Polish Pointers

By Steve Ferranti

Getting the most out of a plastic lens polishing slurry is a critical component to the successful operation of today’s ophthalmic laboratories. Whether a lab has a single machine doing 30 to 40 jobs per day or multiple machines polishing 10,000 lenses per day, a common theme heard from lab managers is how to get the most out of their polish. Lab managers expect a polish to produce a high-quality surface, to clean off easily from the substrate surface, to have a long polish life and to be environmentally friendly.

Manufacturers of today’s plastic lens polishes have invested significant resources in the development of polishes that meet the demanding requirements of today’s ophthalmic laboratories. Polishes contain a complex array of abrasives and chemical additives that enhance polishing removal rates, improve the cleanability of the lenses, minimize foam formation and are environmentally safe. Manufacturers and distributors also provide training and technical support on the proper use of their polishes. By following several basic practices, lab managers and operators can be confident that they are getting the most out of their polish. Here you will find six suggested practices to keep your polish flowing.

Shake, shake and shake again

Before dispensing any polish from its container, thoroughly shake or mix the contents to obtain a homogenous mix. The abrasives often settle out during storage. Operators have been known to pour off the liquid fraction thinking that it is only water and of no use. Similarly, it is a practice in some labs to deliberately pour off the liquid portion to increase the percent solids (abrasive) in the slurry. Both of these practices are detrimental to a polish. The aqueous portion of polishing slurry contains several active ingredients. These components are carefully balanced to achieve optimal polish performance and extend the life of the polish.

Remember—each time a portion of slurry is poured into a sink, you are pouring money down the drain!
Don’t add that antifoam

Antifoam agents in today’s polishes have been designed exclusively for this specific application and to be compatible with the other components in the system. Adding other antifoams (even if recommended by a reputable ophthalmic lab supplier) may compromise the integrity of the polish. As waste products build up in the slurry, foaming may occur. The periodic replenishment of new slurry to the slurry tank will supplement the antifoam and should correct the problem. If foaming continues, it is possible that contaminates such as surfactants from other sources have been introduced into the polish (see below). If foaming is an ongoing problem, contact your distributor or the manufacturer for guidance.

Avoid transferring contaminants to the slurry
Many labs use small amounts of additives in the fining water to improve the fining process. These need to be rinsed off thoroughly from the lenses before polishing. A build up of these additives can result in foaming of the polish itself. In addition, the coolants used in lens generation can also find their way into the polish via transfer to the fining medium, and hence, into the polish. Another critical external polish contaminant is the fining abrasive. Inadequate post-fining rinsing of the lenses can lead to transfer of fining abrasive into your slurry. Small levels of this material can poison a system leading to an increase in scratches and a general reduction in post-polish surface quality. The fining particles are usually too small to be captured by most filtering systems. Therefore, the best way to avoid problems with them is to keep them out of your slurry right from the beginning.

Temperature control

Maintaining temperature control is especially important when polishing polycarbonate lenses. Most labs try to maintain a slurry temperature between 10°C and 15°C (50°F to 60°F). Slurry temperature is usually measured in the tank. However, if the tank is supplying multiple machines, there is a very good possibility that the actual slurry temperature at the point of discharge in the polisher farthest from the tank may have a significantly higher discharge temperature. It is recommended that this last slurry point of discharge temperature be monitored. If too high or low, make the necessary temperature adjustments at the slurry holding tank. It may be necessary to monitor the temperature at several points of discharge to determine the optimal chilling system set point.

Baume

Baume readings are one of the most misunderstood concepts related to plastic polish slurries. When polishing problems are encountered, the first item most operators check is the slurry baume. Slurry baume is a good indicator of the total concentration of components in the slurry. Baume readings should remain relatively stable throughout the slurry life. Over time the baume will drift slightly lower. This is often caused by loss of slurry components (primarily abrasives) due to drag out. Regular slurry replenishment should/will restore the baume to normal levels. Baume readings that differ significantly from the manufacturer’s specification (or normal operation baume) may indicate a loss of abrasive. If this occurs, the first item to check is the filter system. If it is plugged, the filter system can remove significant amounts of abrasive. Should this occur regularly, more frequent cleaning of the filter bag and/or filter housing will be required or a larger mesh filter will need to be employed. If the filter system is not the culprit and no other source of slurry loss can be found, other factors—most commonly entrapped air in the slurry—may be causing the loss of baume. Build up of organic contaminants (e.g., residual lens monomer, waste polymer or drag in waste from the fining operation) can lead to the formation of micro-bubbles in the slurry. This can result in baume readings approaching zero with no subsequent loss of polish performance. Yet many operators will react to the low baume and assume the polish is at fault and discharge the system. If a baume reading is substantially lower than it should be, do not assume that the slurry is no good. Inspect your system and lab procedures carefully. Most times a non-slurry related practice is the cause for the low baume.

Filtration

Effective filtering is crucial to extending the life of slurry and optimizing lens surface quality. Lab operators have utilized a number of filtering systems ranging from an old stocking tied to an outlet pipe to fairly sophisticated inline filter housings complete with bag/cartridge filters supplied by ophthalmic equipment manufactures. More sophisticated filtration schemes employ a two-stage filter system. For single filter systems, 200 microns is the smallest recommended mesh. For two-stage filter systems, the final filter can be 150 microns. Proper filter maintenance is important. Filters can plug rapidly (plugging rate is dependent on filter mesh size, number of jobs per hour and volume of slurry in system). Once a filter plugs, valuable abrasive is removed from the system. This results in reduced polishing rate, diminished surface quality and reduced polish life. The operator will be forced to replenish the slurry system more frequently to make up for the loss of abrasive.

The number of available polishing slurries for ophthalmic lens polishing has grown significantly in the last five years. Each polish has its own unique performance characteristics. Some are more suited for polishing polycarboante and mid- to hi- index substrates. Other polishes work best on more traditional substrates, such as CR-39, while still others are advertised to work on any substrate. Regardless of which polish you use, following the practices noted here should allow you to maximize the performance of your polish.

Steve Ferranti has been an R&D scientist at Ferro Corporation's Electronic Materials Systems since 2001. His primary responsibility is the development of abrasives and abrasive polishing formulations used for fine and ultra fine polishing of surface components used in the optical, ophthalmic and microelectronic industries. Prior to joining Ferro, Ferranti was an R&D scientist for 20 years at Eastman Kodak Company. He can be reached at 315-227-5377 or by e-mail [email protected]